Indicator for proper or improper exposure by automatic electronic flash

ABSTRACT

An indicator for proper or improper exposure provided by automatic electronic flash includes a first decision circuit having a first decision level lower than a proper level of exposure, and a second decision circuit having a second decision level higher than the proper level of exposure. Thus, a given film latitude is allowed for the overexposure and the underexposure in determining and displaying the overexposure, the underexposure or proper exposure.

BACKGROUND OF THE INVENTION

The invention relates to an indicator for proper or improper exposure by an automatic electronic flash, and more particularly, to such indicator which indicates the result of an actual exposure provided by an automatic electronic flash, giving an indication of an overexposure, underexposure or proper exposure.

As is well recognized, when taking a picture with the aid of an automatic electronic flash, the amount of light emission from the electronic flash can be controlled within a limited range. The control of the electronic flash tends to result in an overexposure as a distance to an object being photographed decreases or as a diaphragm aperture increases toward an open value. For example, in an automatic electronic flash of series controlled type, if an emission terminate signal is applied in synchronism with a flash emit signal, there results a finite or minimum emission of light from the electronic flash rather than resulting in zero emission. This is attributable to the nature of a forced commutation circuit including a thyristor of an automatic emission control circuit contained in the electronic flash. If the emission terminate signal causes the thyristor to conduct, it takes a given magnitude of turn-on time which is inherent to the thyristor. Recently, TTL automatic electronic flash is in practical use which is controlled in accordance with an output from an automatic exposure control circuit of a camera of TTL (through-the-lens) direct photometry type. This permits the implementation of a variety of lighting set-ups or the use of multiple electronic flashes in an automatic emission control mode, by interconnecting the camera and the electronic flash or flashes by means of electric cords. However, a transmission lag due to transmission lines running between the camera and the emission control circuit of the electronic flashes or the multiple lighting causes an increase in the minimum emission. For the sake of reference, representing the minimum emission of each individual electronic flash by G_(NO) L, the minimum emission where N electronic flashes are used to provide a multiple lighting will be equal to G_(NO) L×√N. When the minimum emission increases in this manner, an amount of exposure Ev given by actual light emission from electronic flash or flashes will be as indicated by a curve L in FIG. 1. Thus, the amount of exposure will deviate from a proper exposure level L₀ toward the overexposure as the distance to an object being photographed decreases. The overexposure renders it impossible to take a picture with the aid of an electronic flash in a range of distance to an object being photographed which is located nearer than a point P toward the photographer. On the other hand, the maximum emission of the electronic flash will be exceeded with a distance to an object being photographed which is located further beyond a point D, resulting in an underexposure.

A variety of indicators for providing an indication of proper or improper exposure given by an automatic electronic flash are known, including

1. an apparatus for providing an indication of proper or improper exposure based upon the comparison against a first level corresponding to proper exposure and a second level which represents a given amount of overexposure, of an output from an integrator which integrates the amount of light incident (see Japanese Utility Model Publication No. 2,977/1982);

2. an apparatus for providing an indication of proper or improper exposure based upon a determination if an output from the integrator has reached a proper level at time which occurs a given time interval after the initiation of the emission of flashlight (see Japanese Laid-Open Patent Application No. 156,620/1977);

3. an apparatus including means to give a warning of underexposure at a given time interval after the initiation of emission of the flashlight (or closure of X-contacts), and in which the warning means is disabled if the electronic flash has ceased to emit flashlight at a proper exposure level (see Japanese Utility Model Publication No. 19,075/1981).

With the apparatus mentioned above sub-paragraph 1 above, the amount of incident light supplied from the electronic flash is monitored to determine and to indicate if an overexposure level, which is a given value above a proper exposure level, is reached. With this apparatus, while a given latitude from the proper level is considered for the overexposure, no latitude is considered in the determination of the underexposure. Consequently, if the integral of an amount of light incident when the electronic flash is allowed to provide a full emission is less than the proper level by an amount which is as little as 0.1 Ev, the apparatus provides an indication of underexposure. Accordingly, there results an unbalanced criterion of the overexposure and the underexposure. In practice, it will be desirable that a given latitude should be allowed for the underexposure in the same manner as such latitude is allowed for the overexposure. For practical purposes, a satisfactory photographing operation is achieved with a latitude on the order of -1/2 Ev to -1/3 Ev. It should also be noted that the integration of an amount of incident light during a photographing operation with the aid of an electronic flash is normally initiated in synchronism with the initiation of light emission from the electronic flash, and thus a component of an integral which is attributable to steady-state light, though of relatively low brightness, which exists until the first blind of the shutter becomes fully open, is discarded from the determination. To take this into consideration, it is reasonable to allow a given latitude for the underexposure.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the invention to provide an indicator for proper or improper exposure provided by automatic electronic flash in which an integral of an amount of incident light is compared against a pair of decision levels which correspond to a given latitude on the overexposure and the underexposure side and in which an exposure level located between these decision levels is determined to be proper while an exposure level outside the range defined by the decision levels is determined to be either an overexposure or an underexposure.

This improves the indication of the underexposure, as compared with the prior art practice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates the amount of exposure plotted against a distance to an object being photographed with the aid of an electronic flash;

FIG. 2 is a circuit diagram of an indicator for proper or improper exposure provided by an automatic electronic flash, which is constructed in accordance with one embodiment of the invention;

FIG. 3 graphically illustrates the operation of the indicator shown in FIG. 2, also illustrating the decision levels used to determine an overexposure and an underexposure; and

FIGS. 4(a) to 4(e) are a series of timing charts illustrating the waveform of various signals appearing in the indicator of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 2, there is shown an electrical circuit of an indicator for proper or improper exposure provided by an automatic electronic flash which is constructed in accordance with one embodiment of the invention. The indicator is adapted to be incorporated into a camera of TTL direct photometry type and having a photoelectric transducer element PD₁ which is used for purpose of photometry. Light from an object being photographed passes through a taking lens 1 and a diaphragm 2 of the camera to be reflected by a film surface 3 and the surface 4 of a shutter blind. The transducer element is disposed toward the bottom of the camera and is directed upward to receive light reflected by the film surface 3 and the blind surface 4. The transducer element PD₁ has its anode and cathode respectively connected to the non-inverting and the inverting input terminal of an operational amplifier OP₁, with the anode being connected to the ground through an integrating capacitor C₁. The junction between the transducer element PD₁ and capacitor C₁ is connected through a trigger switch SW₁ to receive a reference voltage Vref1. The switch SW₁ is opened in ganged relationship with the initiation of running of the first blind of the shutter. The switch SW₁ is closed in ganged relationship with the termination of a winding operation of the first blind. The output of the amplifier OP₁ is connected to the inverting input terminal thereof, and is also connected through a series combination of voltage divider resistors R₁, R₂ to a source of reference voltage (not shown) which develops reference voltage Vref1. The output of the amplifier OP₁ is also connected to the inverting input terminal of a comparator OP₃ which operates to determine an underexposure, and to the non-inverting input terminal of a comparator OP₄ which operates to determine an emission termination level. The junction between the resistors R₁, R₂ is connected to the non-inverting input terminal of a comparator OP₅ which operates to determine an overexposure.

There is provided another source of reference voltage (not shown) which produces a reference voltage Vref2 higher than the first mentioned reference voltage Vref1, and the reference voltage Vref2 is applied to the non-inverting input terminal of an operational amplifier OP₂ and to the base of each of PNP transistors Q₁, Q₂ and Q₃. The inverting input terminal of the amplifier OP₂ is connected to the collector of the transistor Q₁, and its output terminal is connected to the emitter of each transistor Q₁, Q₂, Q₃. The collector of the transistor Q₁ is also connected to one end of a variable resistor Rv1 which is used to preset film speed information, the other end of the resistor Rv1 being connected to the ground. The collector of the transistor Q₂ is connected through a resistor R₃ to the source of reference voltage Vref1, and is also connected to the non-inverting input terminal of the amplifier OP₃. The collector of the transistor Q₃ is connected through a resistor R₄ to the source of reference voltage Vref1, and is also connected to the inverting input terminals of the comparators OP₄ and OP₅. It is to be noted that the resistors R₃ and R₄ are chosen such that the resistance of resistor R₃ is less than the resistance of resistor R₄ (R₃ <R₄).

The output of the comparator OP₃ is connected to one input of NAND circuit ND₁. The output of the comparator OP₄ is connected to the input of an inverter IN₁. The output of the comparator OP₅ is connected to one input of NAND circuit ND₂. The other input to NAND circuit ND₁ is fed from a first output terminal of a timing pulse controller TPC₁ so as to be supplied with an underexposure examine pulse signal S₁, as shown in FIG. 4(c). The other input of NAND circuit ND₂ is connected to a second output terminal of the timing pulse controller TPC₁, so as to be supplied with an overexposure examine pulse signal S₂, as indicated in FIG. 4(b). The underexposure examine pulse signal S₁ rises from its "L" level to its "H" level at a given time interval on the order of several milliseconds, which corresponds to a full emission of flashlight from the electronic flash, after the closure off X-contacts SW₂ which has its one end connected to timing pulse controller TPC₁ (see FIG. 4(a)). The signal S₁ temporarily remains at its "H" level. The overexposure examine pulse signal S₂ rises from "L" to its "H" level at an overexposure decision time, which is normally on the order of several hundreds of microseconds, after the closure of the X-contacts SW₂, and temporarily remains at its "H" level.

The output of NAND circuit ND₁ is connected to one input of NAND circuit ND₃, which forms an R-S flipflop together with NAND circuit ND₄. Specifically, the output of NAND circuit ND₃ is connected to one input of NAND circuit ND₄, the output of which is connected to the other input of NAND circuit ND₃. Said one input of NAND circuit ND₃ represents a set input while the other input of NAND circuit ND₄ represents a reset input to the flipflop. As shown, the other input of NAND circuit ND₄ which represents the reset input is connected to a fourth output terminal of the timing pulse controller TPC₁, which produces a reset pulse signal S₄ as shown in FIG. 4(e). The reset pulse signal S₄ changes from its "H" to its "L" level at a given delay interval after the closure of the X-contacts SW₂, and reverts to its "H" level after momentarily staying at its "L" level. The output of NAND circuit ND₃, which represents the Q output of the R-S flipflop, is connected to the input of an inverter IN₂, the output of which is connected to the cathode of light emitting diode LD₁ which has its anode connected to receive an operating voltage Vcc in order to indicate an underexposure. The output of NAND circuit ND₄, which represents the Q output of the R-S flipflop, is connected to a first input of a three input NAND circuit ND₇.

The output of the inverter IN₁ is connected to mating electrical contacts (not shown) of the camera and the automatic electronic flash for connection with an automatic emission control circuit (not shown) disposed within the electronic flash. An output from the inverter IN₁ which changes from its "H" to its "L" level is transmitted as an emission terminate signal S₅ to the electronic flash.

The output of NAND circuit ND₂ is connected to one input of NAND circuit ND₅, which forms an R-S flipflop together with NAND circuit ND₆ in the similar manner as the combination of NAND circuits ND₃ and ND₄. The other input of NAND circuit ND₆, which represents a reset input to the flipflop, is connected to the fourth output terminal of the timing pulse controller TPC₁ so as to be fed with the reset pulse signal S₄ therefrom. The output of NAND circuit ND₅, which represents the Q output of the R-S flipflop, is connected to the input of an inverter IN₃, the output of which is connected to the cathode of a light emitting diode LD₂ which has its anode connected to receive an operating voltage Vcc in order to display an overexposure. The output of NAND circuit ND₆, which represents the Q output of the R-S flipflop, is connected to a second input of the three input NAND circuit ND₇

A third input to the three input NAND circuit ND₇ is connected to a third output terminal of the timing pulse controller TPC₁ so as to be fed with a proper exposure display pulse signal S₃ which undergoes a temporary excursion from its "L" to its "H" level in synchronism with the reversion of the underexposure examine pulse S₁ to its "L" level. The output of NAND circuit ND₇ is connected to one input of NAND circuit ND₉, which forms an R-S flipflop together with NAND circuit ND₈, in the similar manner as the combination of NAND circuits ND₃ and ND₄. One input of NAND circuit ND₈, which represents a reset input to the R-S flipflop, is connected to the fourth output terminal of the timing pulse controller TPC₁ so as to be fed with the reset pulse signal S₄ therefrom. The output terminal of NAND circuit ND₉, which represents the Q output of the R-S flipflop, is connected to the input of an inverter IN₄, the output of which is connected to the cathode of a light emitting diode LD₃ which has its anode connected to receive an operating voltage Vcc in order to display a proper exposure.

In operation, upon depression of a shutter release button of the camera, a movable reflecting mirror is resiliently driven out of a taking light path, followed by the initiation of running of the first blind of the shutter. The trigger switch SW₁ is opened in interlocked relationship therewith (see FIG. 3). The incidence of light from an object being photographed which has passed through the taking lens 1 and the diaphragm 2 and reflected by the film surface 3 and the blind surface 4 onto the transducer element PD₁ causes the latter to produce a photocurrent Ip, which charges the integrating capacitor C₁. Accordingly, the voltage across the capacitor C₁ or a potential V₁ applied to the non-inverting input terminal of the amplifier OP₁ increases gradually as indicated in FIG. 3. Because the capacitor C₁ is previously charged to the reference voltage Vref1, the voltage V₁ can be expressed as follows: ##EQU1##

The output voltage V₁ of the amplifier OP₁ is equal to the input voltage V₁. The output voltage V₁ is applied to the inverting input terminal of the comparator OP₃ and to the non-inverting input terminal of the comparator OP₄. The output voltage V₁ is also divided by the voltage divider, whereby a voltage V₂ is developed at the junction between the resistors R₁, R₂ which is expressed as follows: ##EQU2## The voltage V₂ is applied to the non-inverting input terminal of the comparator OP₅.

The transistor Q₁ has its base, emitter and collector connected to the non-inverting input terminal, the output terminal and the inverting input terminal, respectively, of the amplifier OP₂. The amplifier OP₂ operates to maintain the collector potential equal to the reference voltage Vref2 by a collector current, which is expressed as follows:

    Ic=(Vref2/Rv1)                                             (3)

The magnitude of the current Ic is determined in accordance with film speed which is established by an adjustment of the variable resistor Rv1. The same collector current Ic flows through the collector of each of the transistors Q₂ and Q₃ which have their emitters and bases connected in common with the transistor Q₁, and accordingly, collector voltages V₃ and V₄ are developed at the collector of the transistors Q₂ and Q₃ by the collector current Ic which flows through the resistors R₃ and R₄, respectively: ##EQU3## Since resistance of the resistor R₃ is less in magnitude than that of the resistor R₄, the voltages V₃ and V₄ are related such that V₃ <V₄, as indicated in FIG. 3. The voltage V₃ is applied to the non-inverting input terminal of the comparator OP₃ as a reference voltage which is made the basis to determine an underexposure in consideration of film speed, while the voltage V₄ is applied to the inverting input terminals of the comparators OP₄ and OP₅ as reference voltages which are made the basis to determine an emission terminate level and an overexposure, respectively, as the film speed is considered. At the initiation of running of the first blind, both V₁ and V₂ are less than V₃ and V₄, as indicated in FIG. 3, whereby the comparators OP₃, OP₄ and OP₅ produce outputs of "H", "L" and "L" level, respectively.

When the first blind has completed running and the shutter becomes fully open, the X-contacts switch SW₂ is closed. See FIGS. 3 and 4(a). Thereupon, the automatic electronic flash is activated to emit flashlight, through a signal path, not shown, and the flashlight is reflected by an object being photographed to impinge upon the camera. Accordingly, the voltage V₁ which represents an integral of the amount of incident light as well as the voltage V₂ which represents a division of the voltage V₁ both increase rapidly, as indicated in FIG. 3.

Subsequently at a given time interval to determine an overexposure after the closure of the contacts SW₂, the overexposure examine pulse signal S₂ is produced by the timing pulse controller TPC₁. If the voltage V₂ exceeds the reference voltage V₄ at the time the signal S₂ is produced, this implies the existence of an overexposure if a given film latitude is taken into consideration as compared with the decision level at which the emission terminate signal S₅ is produced, as indicated in FIG. 3. Accordingly, the overexposure is displayed. Specifically, at time when the output of the comparator OP₅ changes to its "H" level for V₂ >V₄, and when the signal S₂ momentarily changes to its "H" level, the output of NAND circuit ND₂ momentarily assumes its "L" level, whereby the R-S flipflop formed by NAND circuits ND₅ and ND₆ is set, producing an "H" level output from NAND circuit ND₅. This causes the inverter IN₃ to produce an output of "L" level, whereby the light emitting diode LD₂ is energized to display the overexposure to a photographer. If, on the contrary, the voltage V₂ does not exceed the reference voltage V₄ at the time the signal S₂ is produced, no display of the overexposure is made. Specifically, the output of the comparator OP₅ remains at its "L" level, so that a momentary change of the signal S₂ to its "H" level cannot change the output of NAND circuit ND₂ from its "H" level, thus failing to set the R-S flipflop to energize the diode LD₂.

Subsequently, when a time interval corresponding to a full emission from the electronic flash has passed since the closure of the contacts SW₂, the timing pulse controller TPC₁ produces the underexposure examine pulse signal S₁, as indicated in FIG. 4(c). If the voltage V₁ does not reach the level of the reference voltage V₃ at the time the signal S₁ is produced, this represents an underexposure if a given film latitude is taken into consideration as compared with the decision level where the emission terminate signal S₅ is produced, as shown in FIG. 3. Accordingly, the underexposure is displayed. Specifically, when the signal S₁ momentarily changes to its "H" level while the output of the comparator OP₃ remains at its "H" level, the output of NAND circuit ND₁ momentarily changes to its "L" level, thus setting the R-S flipflop formed by NAND circuits ND₃ and ND₄. Consequently, the output of NAND circuit ND₃ assumes its "H" level while the output of the inverter IN₂ assumes its "L" level, energizing the light emitting diode LD₁ to display the underexposure to a photographer. If, on the contrary, the voltage V₁ exceeds the reference voltage V₃ at the time the signal S₁ is produced, no display of the underexposure is made. Specifically, if the signal S₁ momentarily changes to its "H" level when the output of the comparator OP₃ reverts to its "L" level, the output of NAND circuit ND₁ does not change from its "H" level, whereby the R-S flipflop formed by NAND circuits ND₃ and ND₄ cannot be set to energize the diode LD₁.

Immediately after the underexposure examine pulse signal S₁ is produced, the proper exposure display pulse signal S₃ is produced by the timing pulse controller TPC₁, as shown in FIG. 4(d). It is to be noted that at the time when the signal S₃ is produced, the examination of both the overexposure and the underexposure has already been completed, and the R-S flipflop formed by NAND circuits ND₃ and ND₄ and the R-S flipflop formed by NAND circuits ND₅ and ND₆ are in either their set or reset condition. Assuming that the examination of the overexposure and the underexposure failed to detect either condition, both flipflops are reset, feeding two inputs of "H" level to the three input NAND circuit ND₇. When the third input to the circuit ND₇ momentarily changes to its "H" level in response to the application of the signal S₃ while other two inputs assume the "H" level, the output of NAND circuit ND₇ momentarily changes to its "L" level, thus setting the R-S flipflop formed by NAND circuits ND₈ and ND₉. Thus the output of NAND circuit ND₉ assumes its "H" level and hence the output of the inverter IN₄ assumes its "L" level to energize the light emitting diode LD₃ to display the proper exposure to a photographer. On the other hand, if the overexposure or the underexposure has already been detected when the signal S₃ is produced, either R-S flipflop formed by NAND circuits ND₃ and ND₄ or ND₅ and ND₆ would be set, whereby one of the first and the second input to NAND circuit ND₇ assumes its "L" level. Hence, the application of the signal S₃ to change the third input to its "H" level momentarily does not result in changing the output of NAND circuit ND₇ to "L" level. Thus, the R-S flipflop formed by NAND circuits ND₈ and ND₉ cannot be set to energize the diode LD₃.

Thus it will be seen that by the time the proper exposure display pulse signal S₃ is produced, a determination is given indicating whether the photographing operation performed with the aid of the electronic flash has resulted in an overexposure, an underexposure or a proper exposure, by selectively energizing either one of the diodes LD₁, LD₂ or LD₃. At a given time interval for display after the closure of the X-contacts SW₂, the timing pulse controller TPC₁ produces the reset pulse signal S₄, as indicated in FIG. 4(e), which signal is applied to the other input of NAND circuit ND₄, the other input of NAND circuit ND₆ and one input of NAND circuit ND₈, thus resetting all of the R-S flipflops. Hence, the output of NAND circuits ND₃, ND₅ and ND₉ assume "L" level, deenergizing the light emitting diode LD₁, LD₂ or LD₃ which has previously been energized to terminate the indication of proper or improper exposure provided by the electronic flash.

In the described embodiment, the indicator for proper or improper exposure has been assumed to be disposed within the camera, but it should be understood that it may alternatively be disposed within the automatic electronic flash. 

What is claimed is:
 1. An indicator for proper or improper exposure provided by automatic electronic flash, comprising:means including a first comparison means for producing an emission terminate signal when an amount of exposure provided by light reflected from an object being photographed has reached a proper exposure level which is adjusted to correspond to a preset film speed; a first decision circuit including a second comparison means and having a first decision level which is lower than the decision level corresponding to the proper exposure level; a second decision circuit including a third comparison means and having a second decision level higher than the decision level corresponding to the proper exposure level said proper decision level being intermediate said first and second decision levels so that the difference between the first decision level and the proper decision level is greater than 0.1 Ev; means for displaying an overexposure in response to an output from the second decision circuit at a given time interval after the initiation of emission of flashlight from an automatic electronic flash; means for displaying an underexposure in response to an output from the first decision circuit at a time interval after the initiation of emission flashlight from the automatic electronic flash, the time interval corresponding to the time required for full emission of flashlight from the electronic flash; and means for displaying a proper exposure in response to outputs from the first and the second decision circuits.
 2. An indicator according to claim 1 in which said means for producing the emission terminate signal comprises a comparator having a non-inverting input terminal to which an integral voltage representative of the amount of exposure is applied and an inverting input terminal to which a voltage representing the decision level corresponding to the proper exposure level is applied.
 3. An indicator according to claim 1 in which the first decision circuit comprises a comparator having an inverting input terminal to which an integral voltage representative of the amount of exposure is applied and a non-inverting input terminal to which a voltage corresponding to the first decision level is applied.
 4. An indicator according to claim 1 in which the second decision circuit comprises a comparator having a non-inverting input terminal to which a division of an integral voltage representative of the amount of exposure is applied and an inverting input terminal to which a voltage corresponding to the decision level which corresponds to the proper exposure level is applied.
 5. An indicator according to claim 1 in which the first and the second decision circuits are coupled to gates which are enabled by signals produced by a timing pulse controller to operate the over exposure, under exposure, and proper exposure display means.
 6. An indicator according to claim 1 in which each of said display means is comprised of a light-emitting diode.
 7. An indicator according to claim 1 in which said over exposure, under exposure and proper exposure display means are enabled by signals produced by a timing pulse controller.
 8. An indicator according to claim 7 in which said timing pulse controller develops signals for sequentially enabling said over exposure, under exposure and proper exposure display means in a pre-determined sequence.
 9. An indicator according to claim 1 further comprising a timing pulse controller for generating a plurality of enabling signals;first gating means responsive to a signal from said first decision circuit and one of said timing pulses for operating said under exposure display means; second gating means responsive to a second one of said timing signals and a signal from said second decision circuit for enabling said over exposure display means; and third gating means responsive to the absence of over exposure and under exposure signal conditions from said second and first decision circuits and responsive to a third one of said timing pulses for enabling said proper exposure display means.
 10. An indicator according to claim 9, further comprising flip-flop circuits each coupled between one of said gating means and an associated display means for temporarily storing the detected condition.
 11. An indicator according to claim 10 in which said timing pulse controller further provides a reset pulse for simultaneously resetting all of said flip-flops, a predetermined time after all of said display means have been enabled.
 12. The indicator of claim 1 wherein the difference between said first decision level and said proper exposure level and the difference between said second decision level and said proper exposure level is in the range of from 1/3 Ev to 1/2 Ev.
 13. An indicator according to claim 1 in which the proper exposure display means generates a proper exposure display independently of the emission terminate signal. 